This invention relates generally to a device for measuring torque on a shaft that can turn and, more particularly, to a device for measuring torque making it possible to reduce the inaccuracies which are due to defects in coaxiality of the various constituent parts.
Publication FR 2692986 describes a device for measuring torque on a shaft that can turn, this device comprising two members for generating a magnetic field, these being rotationally integral with a first support and situated in a plane of a right section through a shaft, and two devices for detecting the magnetic field, these being rotationally integral with a second support and immobilized in a plane of another right section through said shaft. The application of a torque to the turning shaft results in a relative angular displacement about the shaft of the detecting devices relative to the devices for generating the magnetic field. This variation is linear with respect to the displacement. In order to maintain this linearity it is necessary to ensure that the gradient of the magnetic field produced by the generating members is as constant as possible in the air gap separating the generating members from the detectors.
In the situation described in document FR 2692986, the field lines of the generating members close through the supports, which are made of ferromagnetic material. To this end, the supports are extended radially beyond the magnets so as to be able to route the field lines situated outside an imaginary cylinder passing through the magnets and coaxial with the shaft. The protective casing which contains the whole may be made of any material, ferromagnetic or otherwise. However, when the radial space is limited as is, for example, the case when the device is intended to be built into the steering gear of a vehicle, or even into the steering column of the vehicle, there is no longer enough space between the magnets and the cover for the magnetic field to be able to close inside the protective housing. Now, it is not acceptable for the magnetic field to close outside the protective housing because in such a case the field and therefore the accuracy of the measurement would depend on the environment outside the housing.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
One approach to try to solve such problems might be to use a housing made of ferromagnetic material, the field thus being closed within the housing at the periphery of the magnets. However, given that the shaft and therefore the magnets attached to it turn with respect to the housing, the slightest dimensional lack of uniformity of this housing or the slightest defect in coaxiality resulting from the way the shaft is mounted in the housing will lead to a variation in the magnetic field, and therefore in the measurement, which variation will not be due to a torque. Thus, a non-zero value for torque will be measured as soon as the shaft rotates, and this will be the case even if no torque is applied.